The present invention relates to motors and their inverter drives. In particular it related to methods and apparatus for connecting polyphase devices.
An alternating current motor is commonly driven by an inverter. An inverter is a device capable of supplying alternating current of variable voltage and variable frequency to the alternating current motor, allowing for control of machine synchronous speed and thus of machine speed. The inverter may also be used with alternating current generators, and can cause an alternating current motor to act as a generator for braking applications. An alternating current motor may be an induction motor, a synchronous motor with either a wound rotor or permanent magnet rotor, or a brushless DC motor.
In many cases, the cost of the inverter is considerably greater than the cost of the motor being supplied. It is thus necessary to minimize the size of the inverter power electronics in order to control system cost.
Whereas the alternating current machine itself may have substantial overload capability, and may carry currents of the order of five to ten times full rated current for periods measured in minutes, the overload capability of the inverter electronics is severely limited. Exceeding the voltage or current ratings of the inverter electronics will swiftly cause device failure. Commonly, inverter electronics is specified such that it can tolerate 150% of nominal full load current for 1 minute, and for any given motor, and inverter will be selected which has the same nominal current capability as that of the motor.
Voltage is set internally by the inverter system or by the rectified supply voltage. Voltage overload is normally not specified, and will cause near instantaneous destruction of semiconductor elements. The voltage ratings of the semiconductors instead set the maximum output voltage of the inverter system, and an inverter will be selected which has a maximum output voltage that matches the operating voltage of the motor at full speed.
With any reasonably sized inverter, substantial motor overload capabilities remain untapped.
Electrical rotating machinery presents impedance that changes with mechanical load and rotational velocity. As the speed of the electrical rotating machine is increased, the voltage produced by a generator, or the voltage required by a motor will tend to increase proportionally. For example, in an induction motor, in order to maintain a constant magnetic field strength as the applied frequency is changed, a constant ratio of applied voltage to frequency is maintained. For permanent magnet machines, the back-emf produced by the motor will increase as rotor speed increases, again requiring increased voltage in order to drive the machine. Even direct current machines (not covered by the present invention) require increased voltage as speed is increased, if magnetic field strength is maintained as a constant.
In general, the required voltage is expressed in terms of Volts/Hertz.
In many traction application, there is limited available electrical power. Thus requirements for high overload capability can only be met at low speed, where high torque is required for starting, but reduced speed means that mechanical power output is still low. Such low speed torque requirements require high current to flow though the motor, but do not require high operating voltage. It is thus possible to trade high speed operating capability for low speed overload capability at the design stage of a motor drive system.
By increasing the number of series turns in the motor windings, higher slot current may be achieved with the same terminal current, thus permitting the same inverter to provide greater overload current to the motor. This increase in overload capability comes at a substantial cost. The increased number of series turns means that the motor operating voltage is increased, operation at high speed is prevented. Most motors are designed for dual voltage operation, through the expedient of operating various subcircuits of the motor in series or parallel connection. The change between series and parallel connection may be accomplished though suitable contactor arrangements, permitting the motor to be operated with a higher number of series turns at low speed, and a lower number of series turns at high speed. For a simple three phase alternating current machine system, such a system would require at least two single-pole three-phase contactors, and would only offer a factor of 1.7 increase in low speed overload capability. With three contactors, a factor of two change is possible.
The change in series turns may be considered a change in alternating current machine impedance, or current versus voltage relation. Normally, an alternating current machine will have a fixed relationship between synchronous speed and impedance, characterized by the Volts/Hertz ratio. For a given inverter and machine frame, a machine wound with a higher Volts/Hertz ratio will have a lower maximum speed, but higher peak low speed torque.
It is thus necessary to provide for an alternating current machine drive system in which the alternating current machine presents a variable Volts/Hertz ratio to the inverter. For high speed operation, the Volts/Hertz ratio would be adjusted to a low value, in order to maintain a suitable alternating current machine operational voltage. For low speed operation, the Volts/Hertz ratio would be adjusted to a higher value, so as to permit high overload torque operation.